Exciton fine structure in strain-free GaAs/Al0.3Ga0.7As quantum dots: Extrinsic effects

We report polarization-resolved high spectral resolution photoluminescence measurements in self-assembled strain-free GaAs/Al0.3Ga0.7As quantum dots designed and realized in order to reduce as much as possible strain and segregation, which affected previous fine-structure splitting (FSS) experiments. Photoluminescence from isolated quantum dots exhibits a linearly polarized FSS. FSS clearly shows a quantum size effect monotonically decreasing from 90 to 20 mu eV by decreasing the quantum dot size (increasing emission energy). While this finding is similar to that observed in strained In (Ga) As/GaAs quantum dots, clearly it requires a different explanation, being our quantum dots not affected by strain-induced piezoelectricity. We ascribed the observed FSS to a size dependent reduction in dot shape anisotropy as evidenced by structural data analysis. Moreover the linear polarization in dots with shape close to cylindrical symmetry is not along the {[110] crystallographic axis but it turns out randomly distributed, highlighting the role of extrinsic effects.}